Nutrient processes and chlorophyll in the estuaries and plume of the Gulf of Papua

This paper investigates the waters of the Gulf of Papua during three cruises of the TROPICS (Tropical River Ocean Processes In Coastal Settings) programme. Plume characteristics were investigated during Leg 1 (May 1997), and estuarine properties during Leg 5a (September 1997) and Leg 7 (January 1999). During Leg 1 the plume was apparent as a well mixed layer up to 30 m deep extending offshore to a distance of 150 km off the Fly River. Lowest salinities were found off the Taruma Delta. Highest chlorophyll concentrations were found at the inner plume close to the river mouth. Dissolved phosphate and nitrate are removed in this zone, whereas silicate behaves conservatively. Primary productivity within the plume appears to rely upon recycled nutrients, with organic fractions representing the majority of the nutrient pool. In the estuaries nutrients were found to behave differently during the monsoon than during the low flow of the extremely dry conditions associated with the 1997 El Niño event. Normally the Fly is characterised by remineralisation of organic nitrogen in the upper estuary, but during drought conditions DON production and NH₄⁺ uptake suggest that bacterial activity is more prevalent. Ocean Colour and Temperature Scanner imagery shows a number of features of the plume, but generally overestimates chlorophyll concentrations due to the effects of high suspended sediment concentrations and, to a lesser extent, coloured dissolved organic matter.